The present invention relates to a station-side terminal apparatus, an optical access network, and a communication method, which are capable of detecting a communication failure due to malfunction of a wavelength-variable optical transmitting and receiving unit of a subscriber-side terminal apparatus, in order to recover communication immediately.
In recent years, a service called FTTH (Fiber To The Home) using the optical fiber as a transmission channel is becoming widespread, for the purpose of providing a broad band service of high speed and wide band to general private homes. For providing the broad band service by FTTH, optical access networks called subscriber passive optical network (PON) are utilized in many places.
In the PON, one station-side terminal apparatus (OLT: Optical Line Terminal) and a plurality of subscriber-side terminal apparatuses (ONU: Optical Network Unit) are connected by branching one optical cable using optical passive elements called optical splitter (optical coupler). In the PON, the FTTH service is provided economically by sharing optical fibers, OLT, etc by a plurality of subscribers.
The PON includes what is called 10G-EPON (10 Gigabit Ethernet (registered trademark) PON) (for example, refer to IEEE (Institute of Electrical and Electronics Engineers) std 802. 3av-2009). In the PON described in this reference, communication from each ONU to the OLT (uplink communication) employs TDMA (Time Division Multiple Access) technology to prevent a collision of signals from each ONU. In what is called TDM-PON, this TDMA technology is used.
Further, in order to meet an increase of communication demand in future optical access network, the research and development relevant to WDM/TDM-PON (TWDM-PON) is progressing, in which a plurality of TDM-PONs are built on one PON infrastructure with WDM (Wavelength Division Multiplexing) technology, as next-generation PON with a transmission rate over 10 Gbps, (for example, refer to JP 2011-55407A). TWDM-PON increases the transmission capacity in the PON infrastructure.
In the TWDM-PON described in JP 2011-55407A, an OLT includes a plurality of optical transmitting and receiving units, and a control device for controlling the TWDM-PON. Each optical transmitting and receiving unit is connected to a plurality of ONUs via optical couplers.
As for uplink communication, the reception wavelength of each optical transmitting and receiving unit of the OLT is fixedly assigned, so that the reception wavelengths of optical transmitting and receiving units of the OLTs do not overlap each other. Then, by changing the transmission wavelength of the optical transmitting and receiving unit of the ONU, the connection between each optical transmitting and receiving unit of the OLT and the ONU is dynamically switched. Also, as for communication from the OLT to the ONU (downlink communication), the connection between each optical transmitting and receiving unit of the OLT and the ONU is dynamically switched by fixedly assigning the transmission wavelength of each optical transmitting and receiving unit of the OLT, and changing the reception wavelength of the optical transmitting and receiving unit of the ONU, in the same way as the uplink communication. Hence, the TWDM-PON has the advantage such as load sharing in response to traffic variation, high reliability by path switching in failure, and electrical power saving by sleep of the optical transmitting and receiving unit and the device circuit during low load.